The entire disclosure of Japanese Patent Application No. 9-319608 including specifications, claims, drawings, and summaries is incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to a color image reading apparatus.
2. Related Background Art
A color image reading apparatus for switching illumination light generally uses three light sources for emitting red (R), green (G), and blue (B) light components, respectively. The wavelength of illumination light is switched by sequentially switching the light source to be turned on.
A color image is reproduced on the basis of R, G, and B image componentswhich have been read using an R, G, and B light sources, respectively.
The conventional color image reading apparatus uses a fluorescent lamp or the like as a light source. Some recent apparatuses use a light-emitting diode as a light source.
Recent light-emitting diodes are quite useful as illumination light sources because of high luminance. In addition, various light-emitting diodes having different emission wavelengths are available.
When light-emitting diodes are used as light sources, three light-emitting diodes, i.e., a red light-emitting diode having a peak emission wavelength in the range longer than 600 [nm], a green light-emitting diode having a peak emission wavelength near 550 [nm], and a blue light-emitting diode having a peak emission wavelength in the range shorter than 500 [nm] are used.
Popular color image reading apparatuses directly output the data of read R (red), G (green), and B (blue) color components. On the other hand, some color image reading apparatuses execute color correction calculation to improve color reproducibility. As the color correction calculation, for example, 3xc3x973 matrix calculation is performed.
The conventional color image reading apparatuses cannot obtain sufficient color reproduction performance when an image is read from an original such as a color reversal film with a wide color gamut. That is, the color image reading apparatuses fail to provide satisfactory color reproduction capability. Especially, when a film having four photosensitive layers is used as an original, the color development characteristics of the film are more complex than the color reproduction capability of the color image reading apparatus. For this reason, the color difference between the actual original image and the read image becomes more conspicuous.
The color reproducibility is somewhat improved by executing color correction calculation. However, the color reproducibility is still poor. Especially, when a film having four photosensitive layers is used as an original, further improvement of color reproducibility is required.
Color image reading apparatuses sequentially read images of the R, G, and B color components. This prolongs the image reading time as compared to monochrome reading.
When the emission intensity of the illumination light source is low, the illuminance of the illuminated original surface is low. To read the image at a sufficient brightness, the exposure time (charging time) of the image sensing device must be made longer. As the exposure time becomes longer, the image reading time also becomes longer.
It is an object of the present invention to improve the color reproducibility of a color image reading apparatus without largely increasing cost. It is another object of the present invention to provide a color image reading apparatus capable of shortening the image reading time.
According to the present invention, there is provided a color image reading apparatus comprising:
a color separation unit to separate a color of an image of an object into not less than four visible color wavelength components in a visible wavelength range;
an image sensing unit to read the image of the object whose color is separated by the color separation unit and outputting image signals of the respective colors; and
a color calculation circuit to calculate image data of not less than three colors from the image signals corresponding to the colors separated by the color separation unit.
The color image reading apparatus according to one mode of the present invention has a light source having four or more types of light emitters which have different peak emission wavelengths in the visible light range, an illumination optical system for illuminating an original with light from the light source, an image forming optical system for forming the image of the original illuminated by the illumination optical system, a one-dimensional image sensing unit for reading the image formed by the image forming optical system as a one-dimensional image, a subscan unit for moving the position of the original relative to the one-dimensional image sensing unit in an axial direction crossing the axial direction of the one-dimensional image, an emission color switching unit for controlling the state of each of the four or more types of light emitters to switch the emission color of the light source, and color calculation means for obtaining three or more color-converted data on the basis of a plurality of color data obtained from the image sensing unit.
In this apparatus, the light source for illuminating the original has four or more types of light emitters which have different peak emission wavelengths in the visible light range. The emission color switching device controls the state of each of the four or more types of light emitters to switch the emission color of the light source. The original is illuminated with light from the light source through the illumination light system. The image of the illuminated original is formed on the one-dimensional image sensing device by the image forming optical system. The one-dimensional image sensing device reads the image formed by the image forming optical system as a one-dimensional image. Since the subscan means moves the position of the original relative to the one-dimensional image sensing means in the axial direction crossing the axial direction of the one-dimensional image, the two-dimensional image of the original can be read.
The color calculation means obtains three or more color-converted data on the basis of a plurality of color data obtained from the image sensing means. For example, when five types of light emitters of R (red), Y (yellow), G (green), C (cyan), and B (blue) are used, the image of each of the R, Y, G, C, and B color components can be sequentially read.
To output color image data having three color components of R, G, and B, which are generally used, image data of three color components of R, G, and B are generated from image data of each of the color components of R, Y, G, C, and B by calculation (conversion) by the color calculation means. The color data of the image generated on the basis of the image data of four or more color components has good color reproducibility as compared to color data obtained by the conventional image reading apparatus. Therefore, even when a film having four photosensitive layers is used as an original, the color image reading apparatus of the present invention can reproduce the color of the original image more accurately.
In this color image reading apparatus, the emission color switching means may have, as a reading mode, a first mode in which the emission color of the light source is sequentially switched to any one of three colors, and a second mode in which the emission color of the light source is sequentially switched to any one of N colors (Nxe2x89xa74). More preferably, the color calculation means automatically may switch the contents of formulas representing the correlation between the input color data of three or more colors and output color data of three or more colors in accordance with the reading mode of the emission color switching means.
When images of four or more color components are to be sequentially read, in some cases, the reading time may become longer than that in reading images of three color components due to more complicated image processing etc. When a film having four photosensitive layers is used as an original, high color reproducibility is required for the color image reading apparatus. However, when the original has a relatively narrow color gamut, it is sometimes more preferable to shorten the reading time rather than to increase the color reproducibility. When the first mode in which the emission color of the light source is sequentially switched to any one of three colors and the second mode in which the emission color of the light source is sequentially switched to any one of N colors (Nxe2x89xa74), the color reproducibility is relatively low while the reading time is relatively short in the first mode. In the second mode, the color reproducibility is improved while the reading time becomes longer.
The color components of input image data change between the first and second modes. The color calculation means automatically switches the contents of formulas representing the correlation between the input color data of three or more colors and output color data of three or more colors in accordance with the reading mode.
In this color image reading apparatus, when the emission color switching means selects a specific color as the color to be emitted by the light source in a specific reading mode, a plurality of light emitters having different peak emission wavelengths may be simultaneously driven to emit light. For example, B (blue) and C (cyan) have a small wavelength difference. For this reason, B and C can be regarded as one color (B). In this case, when the B and C light emitters are simultaneously turned on, the emission intensity of the light source increases as compared to a case wherein only the B light emitter emits light. Similarly, G (green) and Y (yellow) have a small wavelength difference. For this reason, G and Y can be regarded as one color (G). In this case, when the G and Y light emitters are simultaneously turned on, the emission intensity of the light source increases as compared to a case wherein only the G light emitter emits light. As the emission intensity of the light source increases, the illuminance of the original increases. For this reason, even when the exposure time of the image sensing device is shortened, a bright image can be read. That is, the image reading time can be shortened.